1. Field of the Invention
The present invention relates to an optical film with functions of light polarization, light collimating and limiting viewing angle for a liquid crystal display.
2. Description of Related Art
In current LCD (Liquid Crystal Display) technology, only 4-6% of the total luminance provided by back light source is escaped from the viewing side of a panel. The low light energy efficiency is mostly caused by the LCD elements of color filters and dichroic polarizers. Especially, the element of polarizer, in principle, absorbs half of the light energy that provided by back light unit of a LCD. However, there's still room for improving the light efficiency by changing the polarizer materials.
Accordingly, many solutions were proposed to solve this problem. One of the most efficient way is using a reflective polarizer which can recycle and reuse the polarized light which was supposed to be absorbed by a polarizer. On the other hand, some displays mainly for personal use such as notebook computers, mobile phones, desktop monitors, ATM machine, and so on, do not require large viewing angle but they need privacy, still require brightness enhancement. Therefore, for those applications, it is only necessary to enhance brightness around the center of the displays with small viewing angle and are currently achieved by using a special collimating film with specific structures thereon, for example, Lumisty® film (Sumitomo Chemistry Inc).
To achieve functions of light polarization, light collimating and limiting viewing angle for a liquid crystal display at the same time, it is used to apply many films to collect all those functions. The present invention intends to achieve all the functions by using only one piece of new optical film.
A novel way to polarize light for enhancing brightness is to utilize a reflective cholesteric liquid crystal film. Therein, non-polarized incident white light is separated into left and right circularly polarized light through the optical chirality and the helical molecular structure of cholesteric liquid crystals. 50% of the circularly polarized light having opposite chirality is transmitted, while the other 50% having the same chirality is reflected by the cholesteric liquid crystal film. The reflected light can be re-directed into the panel by the layers of back light unit. Theoretically, all the light from a back light unit can be transformed into one of the circular polarized light. It is kind of lossless during this light polarization step. Furthermore, a quarter-wavelength retardation plate can be used to transform circularly polarized light into linearly polarized light and its polarization axis is possible to be arranged at the same angle of the dichroic polarizer. Therefore, all of the light is able to pass through the dichroism polarizer, thereby the brightness is extremely enhanced.
Conventionally, the light enhancing film made by cholesteric liquid crystals comprises a single layer or multiple layers. No matter how many layers it has, the selected reflection range (or polarization separation range) of the final product should cover the wavelength of visible light range, especially, the wavelength of the three primary colors of red, green and blue.
The method for preparing a single layer of cholesteric liquid crystal film is disclosed in, e.g. U.S. Pat. Nos. 5,506,704, 5,691,789, 6,099,758, 6,057,008, and 6,071,438. In order to vary the helical pitch of cholesteric liquid crystals, most of these methods utilize long time exposure under low illuminance, difference of reactivity, or further addition of certain dyes, so as to make the effective range of reflective wavelength cover majority of the wavelength of visible light.
One can also make broadband cholesteric liquid crystal layers by multilayer methods. Since the wavelength range of a common single layer of a cholesteric liquid crystal film is limited, a plurality of layers of a cholesteric liquid crystal film are prepared by coating multiple times such that the wavelength range of each layer can be combined together so as to cover the whole wavelength range of visible light. For example, U.S. Pat. No. 6,016,177 discloses a plurality of layers of a cholesteric liquid crystal film formed by coating a plurality of layers of cholesteric liquid crystal polymers having different helical pitches on a substrate. The property of such films is that the cholesteric liquid crystal layer is directly coated in a layer-by-layer manner, and the alignment of next layer is done by the previously aligned layer. However, to prepare cholesteric liquid crystal layers by directly multilayer coating method has many drawbacks. Firstly, there are multiple alignment interferences occurring between the layers which will adversely affects the polarizing effect. Secondly, the coating process has to be operated under restrictive conditions, for example, the surface tension of each layer should be precisely controlled to prevent the coating defects. Furthermore, the performance of the final film product is highly susceptible to the process temperature.
In addition, the method for enhancing brightness of a liquid crystal display within small viewing angle, i.e. the well-known “light collimating” technique, is disclosed in U.S. Pat. Nos. 5,136,480 (1992), 5,467,417 (1995), and 5,919,551 (1999). The method involves forming a microstructure on a general transparent polymer film or plate such that light with large incident angle can be guided to the central viewing angle area of the liquid crystal display, thereby the brightness at the central viewing angle area is quite enhancing. However, such approach only relates to light collimating but not light polarization. More than half of the light will still be lost when light transmitted through a polarizing film at the next station on the way to LCD panel. And actually, the total brightness is not enhanced at all due to the additional layers of films and the total reflection. Therefore, although the central viewing angle area of the liquid crystal display looks brighter, the total light transmittance is actually lowered.
The present invention has been carefully studied. It provides an optical film comprising a cholesteric liquid crystal film, which comprises cholesteric liquid crystal layers and by specific combination thereof, the resulting integrated optical film can simultaneously achieve functions of light collimating, light polarization for enhancing brightness and limiting viewing angle.